There is considerable interest in the semiconductor industry in optically monitoring wafers during the fabrication process. Various metrology tools have been developed to characterize thin film layers formed on silicon substrates. Parameters of particular interest include thin film thickness, index of refraction and extinction coefficient.
Recently, a need has developed for measuring very thin gate dielectrics. These gate dielectric layers are typically only about 20 angstroms thick. In order to measure these layers accurately, very stable and repeatable systems are necessary.
One difficulty associated with measuring such thin layers relates to errors associated with variations in temperature of the sample, especially for real-time and/or in situ applications where the sample temperature may vary over a wide range. More specifically, characteristics of the thin dielectric, such as its thickness, are not measured directly from the optical data but must be determined via model calculations. The results obtained depend on the values of many parameters assumed in the model, such as the substrate index of refraction and extinction coefficient. Unfortunately, the substrate index of refraction and extinction coefficient depend in turn on the temperature of the substrate. Hence, if the substrate temperature is not accurately known, the thickness calculation cannot be accurately performed. In fact, the thickness determination can be affected by unknown temperatures variations by as much as 0.01 angstroms per degree centigrade. Because wafers undergo wide temperature variations during processing, a simultaneous knowledge of the temperature of the wafer is critical for accurately and repeatably determining the thickness of the dielectric layer.
One simple method of dealing with this problem is to allow the wafer to cool to room temperature prior to measurement. However, this approach is time consuming. It would be far better to be able to determine the temperature of the wafer directly and take that temperature into account when evaluating the film thickness. Moreover, since the wafer might not cool uniformly or reproducibly, it would be very desirable to know the temperature of the wafer at the point at which the thickness is being measured.
In the prior art, it has been recognized that ellipsometric measurements can provide information simultaneously about the temperature and oxide thickness of a sample. Such work is reported in “Ellipsometric Monitoring and Control of the Rapid Thermal Oxidation of Silicon,” Conrad, et. al, Journal of Vacuum Science Technology B 11(6) November/December 1993. In this paper, the authors describe using a rotating analyzer ellipsometer (RAE) with a fixed compensator to take measurements from which the ellipsometric parameters ψ and Δ can be calculated. From this information, the authors were able to derive information about temperature and film thickness.
A conventional RAE of the type used by the authors includes a polarizer that is rotated at an angular velocity omega (ω). Output signals at twice the rotation frequency (2ω) are generated in both sine and cosine phases.
An RAE is an incomplete polarimeter, i.e., one that cannot measure all three Stokes parameters. As conventionally operated, i.e., without a compensator, the RAE cannot measure S3, which describes circularly polarized light. Unfortunately, under the most favorable operating conditions, i.e., using HeNe laser illumination, the thickness of very thin oxide layers on Si affects only S3. To overcome this limitation, a quarter-wave plate is used to convert the circularly polarized component to linear polarization, to which the RAE can respond. This is the approach adopted by Conrad et al. While this strategy allowed Conrad et al. to determine thickness and temperature simultaneously, being an incomplete polarimeter the RAE remains susceptible to systematic errors, such as depolarization, that could be detected in a complete polarimeter such as a rotating compensator ellipsometer (RCE). A second class of systematic error that can affect an RAE but not a RCE is connected to the fact that the detector must measure an intensity whose polarization is continuously changing with time, as a result of the rotating polarizer.
Accordingly, it would be highly desirable to provide a measurement system which could simultaneously and independently measure both temperature and thickness while avoiding the systematic errors that can occur with a RAE.